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Loss of Brake Redundency

Yahoo Message Number: 89949
Hi Folks,

Ref msg. 89705

Some background first.

The front brakes are feed by the 2 secondary tanks and the hoses are plastic red and the red needle on the dash air pressure gauge responds with brake application.
The rear brakes are feed by the left front primary tank only and the hoses are plastic green and the white needle on the dash air pressure gauge responds with brake application.
This dual configuration is required by federal law so that loss of either air tank(s) will not mean a loss in braking. However, your stopping distance will be increased.

Next.

There are 3 drain valves in the front on the driver's side.
The outboard valve drains the left front primary tank.
The middle valve drains both of the secondary tanks.
The inboard valve drains the right rear primary tank. This tank is fed by the air dryer and then through check valves feeds the left front primary tank and the 2 secondary tanks which are connected in series. The right front primary tank and the right front secondary tank both feed a double check valve which in turn feed the brake pedal valve.

Now for the test.

With the engine running in neutral, pump the brake pedal slowing several times until the governor kicks in below about 100psi and the pressure rebuilds and purge valves operates.
Then turn the engine off.

Open the middle valve to empty the secondary tanks.
Only the red needle should slowly go to zero.
The test can be repeated this time opening the outboard valve draining the left front primary tank and the white needle should only go to zero.

If both needles go to zero, then the double check valve has failed.
This means that loss of pressure in either the left front primary tank or the secondary tanks, all braking is lost.
The double check valve (D20 pneumatic drawing D15993) is the most critical component in the air braking system and should be check periodically.

Fred Kovol

2000 Allure 30593